This application claims the priority of Korean Patent Application No. 2004-108945, filed on Dec. 20, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present relates to a permanent magnet synchronous motor and, more particularly, to a lead-angle control method and device for speed control in flux weakening regions.
2. Description of Related Art
In recent years, permanent magnet synchronous motors (PMSMs) have been widely used in a variety of industrial applications due to their high efficiency characteristics and high torque characteristics over an inertial force. There is an advantage in that the PMSMs can be controlled at high speeds in a constant power region as well as a constant torque region due to their structural characteristics. Along with the advantage, as the PMSMs are required to be operated at higher speeds than a rated speed in most of industrial application fields employing the PMSMs, there have been proposed various control algorithms intended to increase PMSM operation regions within a fixed output range of an inverter. A flux weakening control algorithm is one of them typically employed as such control algorithms.
In general, as the speed of a motor increases, a counter-electromotive force is also increased. The increased counter-electromotive force partly offsets an input voltage. Consequently, it causes a current applied to the motor to be reduced, which limits the maximum speed of the PMSM. It is a primary principle of flux weakening control to apply negative values to a d-axis current to suppress the increased counter-electromotive force.
Meanwhile, a flux weakening control system is operated along a maximum torque-per-ampere trajectory within output voltage/current limit regions to achieve high efficiency. However, in such a flux weakening control system operated along an optimum trajectory, as shown in FIG. 1, there is a problem in that it is not possible to move from a point A to a point B in the current/voltage limit region, which makes higher-speed operation impossible. To overcome this problem, there has been introduced a new speed control method called a lead-angle compensation method which allows for higher-speed operation in the flux weakening region.
A well-known lead-angle control method in a flux weakening region will now be described with reference to FIG. 2.
According to the well-known lead-angle control method in a flux weakening region, the magnitude of an instruction current ise* is first obtained through a speed controller 10, and then rotating-coordinate-system instruction current components idxe*,idxe* for producing maximum torque-per-ampere are obtained from the following Equations 1 and 2:
                              i          qx                      e            *                          =                              sign            ⁡                          (                              i                s                                  e                  *                                            )                                ⁢                                    (                                                i                  s                                      e                    ⁢                                          *                      2                                                                      -                                  i                  ds                                      e                    ⁢                                          *                      2                                                                                  )                                                          [                  Equation          ⁢                                          ⁢          1                ]                                          i          dx                      e            *                          =                                            Φ              f                        -                                                            Φ                  f                  2                                +                                  8                  ⁢                                                            (                                                                        L                          q                                                -                                                  L                          d                                                                    )                                        2                                    ⁢                                      f                    s                                          e                      ⁢                                              *                        2                                                                                                                                      4            ⁢                          (                                                L                  q                                -                                  L                  d                                            )                                                          [                  Equation          ⁢                                          ⁢          2                ]            
The rotating-coordinate-system instruction current components iqxe*,idxe* obtained from Equations 1 and 2 are applied to a voltage modulator 30 and then converted to rotating-coordinate-system instruction voltage components through two PI controllers. Subsequently, the rotating-coordinate-system instruction voltage components are converted to two-phase stationary-coordinate-system instruction voltage components Vdss*,Vqss* through a coordinate converter and then applied to a space voltage vector controller (SVM). The SVM converts the applied stationary-coordinate-system instruction voltage components according to a space vector pulse width modulation method and outputs it to an inverter 40. A motor starts to be operated by a phase voltage applied by the inverter 40.
Meanwhile, the stationary-coordinate-system instruction voltage components Vdss*,Vqss* outputted from the coordinate converter is fed back to a current modulator 20 and then added to a maximum output limit voltage
            V              D        ⁢                                  ⁢        C                    3        .If the magnitude √{square root over ((Vdse*2+Vqse*2))} of the instruction voltage is less than the maximum output limit voltage
            V              D        ⁢                                  ⁢        C                    3        ,the speed control in the flux weakening region is not necessary. Otherwise, a larger negative d-axis current is necessary to offset a counter-electromotive force.
Accordingly, the above-mentioned prior art lead-angle control method performs a speed control operation in such a manner that when the magnitude √{square root over ((Vdse*2+Vqse*2))} of the instruction voltage exceeds the maximum output limit voltage, the counter-electromotive force is suppressed by adding a negative d-axis current Δidf, which is generated by performing a proportional integral operation on a difference between the magnitude √{square root over ((Vdse*2+Vqse*2))} of the instruction voltage and the maximum output limit voltage, to the d-axis instruction current, and a q-axis instruction current is compensated as much as the added amount of the d-axis current.
As apparent from the above description, since the above-mentioned prior art method uses a direct current (DC) link voltage for the lead-angle (speed) control in the flux weakening regions, speed-control performance is dependent on the DC link voltage. As a result, since the DC link voltage usually contains ripple components as shown in FIG. 3, there is a problem in that the speed control may be inaccurate due to erroneous compensation of the d-axis current Δidf, resulting in a decrease in system stability or reliability.